1. Field of the Invention
The present general inventive concept relates in general to a carrier and symbol timing recovery apparatus usable with a Vestigial Side Band (VSB) receiver and a recovery method thereof. More specifically, the present general inventive concept relates to an apparatus to recover carrier and symbol timing more effectively by using a pilot signal, an upper side band, and a lower side band of a VSB signal, and a recovery method thereof.
2. Description of the Related Art
In order for a data receiver that receives data transmitted according to Vestigial Side Band (VSB) modulation mechanism to accurately demodulate the VSB-modulated data, it is necessary to minimize frequency offset and high levels of phase noise (jitter) generated from a tuner or an RF oscillator used for data reception. This procedure is called ‘carrier recovery.’
In order to receive the data more accurately, the data receiver should be able to generate a clock signal that is the same as a clock signal used in a data transmitter. This procedure is called ‘symbol timing recovery.’
A digital broadcast system based on the VSB modulation method in conformity with American digital television standards of the Advanced Television Systems Committee (ATSC) utilizes a pilot signal contained in a transmitted signal for carrier synchronization. The pilot signal is a signal that is loaded on a carrier for transmission such that the carrier can be accurately recovered at the data receiver.
Typically, symbol timing recovery methods use data segment sync signals that are regularly inserted by the data transmitter during the data transmission, or a Band Edge Component Maximization (BECM) algorithm.
FIG. 1 illustrates a frequency spectrum of a VSB signal including a pilot signal.
The frequency spectrum of FIG. 1 is a generic VSB signal having the pilot signal transmitted to a baseband. The frequency spectrum of a digital broadcast signal defined by the ATSC standard is typically the same as the frequency spectrum illustrated in FIG. 1, except that a bandwidth thereof is approximately 6 MHz.
In FIG. 1, ‘a’ represents an upper sideband of a received signal for use in the BECM algorithm, and ‘b’ represents the pilot signal of the received signal.
FIG. 2 is a schematic block diagram illustrating a conventional digital broadcast receiver.
Referring to FIG. 2, the conventional digital broadcast receiver 200 includes an Analog to Digital Converter (ADC) 201, a multiplier 203, a matched filter 205, a carrier recovery circuit 230 that uses the pilot signal, and a symbol timing recovery circuit 250 that uses the BECM algorithm.
The carrier recovery circuit 230 includes a pilot detector 231, a phase detector 233, a loop filter 235, and a Numerically Controlled Oscillator (NCO) 237.
The received signal is digitized by the ADC 201, and is output as a baseband signal through the multiplier 203.
The pilot detector 231 of the carrier recovery circuit 230 detects the pilot signal from the baseband signal, and the phase detector 233 reads phase error information of the pilot signal. Methods for reading the phase error information differ depending on the application. The phase error information acquired by the phase detector 233 is provided to the loop filter 235, is converted to a frequency component by the NCO 237, and is multiplied by the received signal in the multiplier 203.
The carrier recovery circuit 230 repeats the above-described procedure until a phase error of the pilot signal, which is indicated by the phase error information, is reduced to zero.
The symbol timing recovery circuit 250 includes a BECM section 251, a loop filter 253, and an NCO 255.
The baseband signal generated by the multiplier 203 is provided to the matched filter 205, is filtered by the matched filter 205, and is input to the symbol timing recovery circuit 250.
The BECM section 251 outputs error information from a symbol clock from a signal that is input thereto (i.e., an output signal of the matched filter 205) to the loop filter 253, and the loop filter 253 outputs a control voltage that corresponds to the error information to control the NCO 255. Next, the NCO 255 adjusts a sampling clock input to the ADC 201 according to the control voltage received from the loop filter 253.
For accurate demodulation of the received signal, it is very important that both the carrier recovery and the symbol timing recovery are smoothly and properly performed. If one of them fails, it can be impossible to demodulate the received signal accurately.
In practice, however, a lot of noise coexists in a wireless environment or in channels over which signals are broadcasted. Therefore, a signal having the frequency spectrum that is similar to the frequency spectrum illustrated in FIG. 1 is not typically received. Additionally, both the pilot signal and the upper sideband signal of the received signal can be substantially corrupted by the noise.
The corrupted pilot signal indicates that the carrier cannot be recovered properly from the received signal. This causes a deterioration in data receiving performance of the system having the conventional digital broadcast receiver.
Moreover, in some cases, a received VSB-modulated signal is distorted on a transmission channel and thus, the upper sideband thereof is substantially corrupted. This also results in the performance degradation of the conventional digital broadcast receiver and the corresponding system. In particular, the corruption of the upper sideband occurs more often in a multi-path environment, thereby causing deteriorations in the data receiving performance.
Digital broadcast systems based on the VSB method are not completely exempted from the deteriorations in the data receiving performance and the performance degradations of the carrier and symbol timing recoveries due to the corruption of the received signal.